1. Field of the Invention
The present invention relates to a method of controlling movement of a mobile robot, and in particular to a method of controlling movement of a mobile robot which moves along with an accompanied target.
2. Description of the Related Art
Conventionally, there have been introduced various technologies relating to an accompanying method for a mobile robot represented by an automatic cart, an automatic moving vehicle and the like, to follow an accompanied target.
For example, there has been introduced an accompanying method for performing an accompanying behavior by calculating a relative distance and relative velocity between a mobile robot and a leading accompanied target using a wireless device such as an ultrasonic sensor and a wired device such as a lead wire, and by moving the mobile robot to have the relative distance and relative velocity within a predetermined range (e.g. refer to Japanese Laid-Open Patent Applications No. 6-274223, No. 6-105938, and No. 2003-92749).
The conventional mobile robot is described with reference to FIG. 1 and FIG. 2. FIG. 1 is a diagram showing a structure of the conventional mobile robot. FIG. 2 is a diagram showing an accompanying method for use in the conventional mobile robot.
As shown in FIG. 1, a conventional mobile robot 11 is a robot which moves along with an accompanied target 10, and includes: mobile mechanisms 11a such as wheels; a measurement apparatus 11b which detects a relative positional relationship between the mobile robot 11 and the accompanied target 10; a calculator 11c which calculates a path to follow using a measured value by the measurement apparatus 11b; and a control apparatus 11d which controls the mobile mechanisms 11a based on the calculation result by the calculator 11c. 
Here, a measurement method by the measurement apparatus 11b for measuring a relative positional relationship between the mobile robot 11 and the accompanied target 10 is described. In Japanese Laid-Open Patent Application No. 6-274223, the measurement apparatus 11b measures a distance to the accompanied target 10 by transmitting and receiving ultrasonic waves and radio waves to/from a transmitting/receiving apparatus attached to the accompanied target 10.
Also, in Japanese Laid-Open Patent Application No. 6-105938, the measurement apparatus 11b measures a distance and a direction to the accompanied target 10 in accordance with a direction of the lead wire connected to the accompanied target 10, and the like.
Further, in Japanese Laid-Open Patent Application No. 2003-92749, the measurement apparatus 11b measures the direction of the accompanied target 10 by detecting a direction of radio waves transmitted from a portable radio wave transmitter equipped to the accompanied target 10.
In the methods described above, the accompanying movement of the mobile robot 11 to follow the accompanied target 10 is controlled as shown in FIG. 2, based on a measured value obtained by measuring a relative positional relationship between an accompanied target 10 and the mobile robot 11 through wired or wireless means. In FIG. 2, it is assumed that the mobile robot 11 has a positional relationship with the accompanied target 10 as indicated by a straight line 12, and moves at a moving velocity VT. Herein, the accompanying behavior is performed by the control apparatus 11d of the mobile robot 11 which controls the mobile mechanisms 11a so that the mobile robot 11 moves at an accompanying velocity VR toward a same direction as the accompanied target 10, or so that the distance to the accompanied target 10 becomes constant in some cases.
Furthermore, Japanese Laid-Open Patent Application No. 10-172099 discloses a technology relating to a control of an automatic moving vehicle which moves along a moving path of an accompanied target (a leading vehicle).
An example of the case where the control method of the automatic moving vehicle is applied to the mobile robot is described with reference to FIG. 3 and FIG. 4. Here, FIG. 3 is a diagram showing a structure of the mobile robot. FIG. 4 is a diagram showing an accompanying method for use in the mobile robot. Note that, the same reference numbers denote the same constituents as of the mobile robot 11 shown in FIG. 1, and the detailed explanations thereof are omitted here.
As shown in FIG. 3, a mobile robot 21 is a robot which moves along with an accompanied target 10, and includes: mobile mechanisms 11a such as wheels; a measurement apparatus 11b which detects a relative positional relationship between the mobile robot 21 and the accompanied target 10; a path database 11e in which the measured value from the measurement apparatus 11b is stored as information of a moving path 13; a calculator 11c which calculates a path to be followed by the mobile robot 11 itself from the path information of the accompanied target 10 stored in the path database 11e; and a control apparatus 11d which controls the mobile mechanisms 11a based on the calculation result by the calculator 11c. 
As shown in FIG. 4, the mobile robot 21 moves along with the accompanied target 10 which moves at a moving velocity VT. For example, when the accompanied target 10 moves on the moving path 13, the mobile robot 21 controls itself so that it moves along the moving path 13 at an accompanying velocity VR, and follows the accompanied target 10. In particular, the technology disclosed in Japanese Laid-Open Patent Application No. 10-172099 realizes a high-precision accompanying behavior by estimating a path for the mobile robot 21 in a current moving condition compared to the moving path 13 of the accompanied target 10, and by controlling the movement so as to correct the difference between the paths.
However, in the case where the conventional technology is applied to the mobile robot, there are the following problems.
In other words, in the conventional technology, the accompanying behavior is performed so as to follow the current accompanied target or its moving path. Accordingly, this results in the mobile robot following behind the accompanied target. Therefore, when the accompanied target suddenly stops, there is a possibility that a collision with the accompanied target will occur due to a delay of braking by the robot.
Additionally, in the case where the accompanied target is a person, in order to check an accompanying condition of the mobile robot positioned behind the person, the checking must be done by turning around the neck or the body so that the checking procedure requires significant effort in labor and time. Also, there is a possibility that an accident will be caused by the person when turning to check the mobile robot.
The aforementioned problems of the conventional technology are described with reference to FIG. 5 and FIG. 6. Here, FIG. 5 is a diagram showing an example relating to a collision with the accompanied target. FIG. 6 is a diagram showing an example relating to the checking of the accompanying condition of the mobile robot.
For example, as shown in FIG. 5, in an ordinary accompanying condition, the mobile robot 11 (21) is positioned behind the accompanied target 10 and moves in a direction toward the accompanied target 10. Here, when the accompanied target 10 suddenly stops, there is a possibility that the mobile robot 11 (21) will collide with the accompanied target 10 positioned forward because of the delay of braking by the mobile robot 11 (21) if sufficient braking is not applied.
Further, as shown in FIG. 6, in the case where the accompanied target 10 is a person that checks the mobile robot 11 (21) which follows behind, the person needs to turn his/her neck around until he/she can view the area behind or to turn his/her body around so as to face the mobile robot. Herein, there is a possibility that an accidental collision with an obstacle 51 on the way will occur because the person only cares about checking the mobile robot 11 (21) positioned behind.